1. Field of the Invention
This invention relates generally to processing of resin material for fabrication into finished articles and more particularly to methods and apparatus for furnishing liquid color material used to impart a desired color to the finished plastic part.
2. Description of the Prior Art
When liquid color is used to impart a desired color to finished parts produced by molding or extrusion, the liquid color is normally introduced into the process at the throat of the process machine, along with the flow of plastic resin or pellets. In this context the “throat” of the process machine refers to the position at which the plastic resin is initially introduced into a barrel surrounding one or more extrusion screws, if the process machine is a extruder, or the position at which the plastic resin is initially introduced into the screw barrel housing, if the process machine is an injection molding press.
At the throat of the process machine, whether it be an extruder or a molding press, the tube carrying the liquid color from the liquid color supply can simply be placed in location and the liquid color metered or dripped into the flow of the plastic resin material to be processed. No significant liquid color pressure is required for the liquid color to be introduced into the process and combined with the plastic resin or pellets at the throat of the process machine.
It is advantageous to introduce the liquid color further along in the process, i.e. downstream, closer to the position at which the finished plastic parts are produced. By introducing the liquid color further along in the process, namely closer to the position where the final plastic parts are produced, this reduces clean out time and reduces color changeover time.
When colors are changed, some of the coloring agent, whether the agent be liquid color or solid color pellets, is always lost due to the time required to purge the old color from the process machine and to introduce the new coloring agent at full strength. Reduced color changeover time translates into savings in time and reduction in the amount of coloring agent, namely liquid color, that is always unavoidably lost. This can be a very significant cost saving to entities operating numerous extruders and injection molding machines to produce many finished, colored parts.
If liquid color is introduced into the barrel of a extruder screw, downstream from the throat, there is no color at the throat. Hence, there is no color contamination at the throat and color changeover can take place in much less time.
A problem with this approach, namely introducing liquid color into the extrusion screw barrel downstream of the throat, is that pressure in the extrusion screw barrel must be overcome by the metering pump delivering the liquid color. Pressure required to overcome the internal pressure within the extruder screw barrel is between four hundred (400) and fifteen hundred (1500) psi, depending on the location at which the pressure is measured and the particular extruder involved.
Previously, pumps have been used with limited success to supply liquid color directly into the barrel of an extruder screw. Gear pumps have been used, which have the advantage of allowing control of metering rates by simply varying the speed of the gear pump. Varying pump speed, coupled with the high pressure involved in delivering the liquid color, has resulted in the some use of gear pumps to deliver liquid color since gear pumps are “positive displacement pumps”, i.e., gear pumps inherently produce the high pressure required to supply liquid color material directly into the barrel of an extruder screw.
However, gear pumps all have closely fitting, fine tolerance parts, namely the meshing gears. This is a disadvantage because many color pigments are abrasive. White color pigment, which is titanium dioxide, is extremely abrasive and is similar to finely ground stone. Using a gear pump to feed color pigment as a component of liquid color, particularly white liquid color with its very fine particles of titanium dioxide, wears the gears in a gear pump to an unacceptable point in a very, very short time.
A further disadvantage of gear pumps is that they are very expensive.
These factors have kept high pressure injection of liquid color into the barrel of an extruder screw or into the barrel of an injection molding machine screw, at a position downstream from the throat, from being a practical process for operators of injection molding machines and extruders producing finished colored plastic parts. Processors simply have not embraced the introduction of liquid color into the barrels of extruder screws or injection molding machines at positions removed from the throat.